Amplitude modulators for millimeter waves



Sept. 2 3, 1958 H. LEBOUTET AMPLITUDE MODULATORS MILLIMETER WAVES Filed May 19, 1954 2 Sheets-Sheet P 3, 1958 H. LEBOUTET 2,853,688

AMPLITUDE MODULATORS MILLIMETER WAVES Filed May 19,- 1954 2 Sheets-Sheet 2 United States Patent O AMPLITUDE MODULATORS FOR MILLINIETER WAVES Hubert Leboutet, Paris, France, assignor to Compagnie generale de Telegraphie Sans Fil, a corporation of rance Application May 19, 1954, Serial No. 430,954

Clains priority, application France May 22, 1953 4 Claims. (Cl. 333-81) The present invention relates to a device whose function is to interrupt or chop at a given rate, i. e. to amplitude modulate to 100%, a millimeter wave propagating in a wave guide.

As is known, commonly utilized amplitude modulating means cannot be used for millimeter waves. For the latter waves, special modulators are used and, among others, mechanical modulators.

Thus the problem arises in the field of millmeter waves to provide effective attenuators which may be inserted without difliculty in waveguides to amplitude modulate the ultra-high frequency energy. Now, it is impossible to transpose to this field all the arrangements used in the field of centimeter waves as the waveguides are of very small size, ranging about one millmeter.

One is thus led to use only attenuators of the shutter type. It is known to realize such attenuators in the shape of plane cams, made of an absorbing material, which are caused to rotate in the plane of symmetry of a guide, the cams penetrating into the guide through a slot provided in one of its walls.

However, owng to the material used for making such shutters (for instance cardboard coated with carbon or aquadag), the thickness of the latter can hardly be made lower than millmeter. Now, a slot of this width radiates a substantal part of the energy being propagated iu the guide. This is a very serious drawback, in View of the low power supplied by usual millimeter wave generators. i

Attempts have been made to make metal shutters coated with an absorbing material, of much smaller thickness (of the order of millmeter). It has been found however that, for various reasons, the attenuation brought about by such shutters was a poor one.

The present invention relates to a millimeter wave attenuator, of the type mentioned above, which is free of these drawbacks and is more particularly designed to 100% modulate millimeter waves.

The device according to the invention comprises a portion of a Waveguide for millimeter waves, combined with a plane metal Shutter of very small thickness, the shutter being adapted for penetrating, at least in certain positions thereof, into'the guide substantially in the middle plane Ofthelatter. That portion of Shutter, which penetrates into the guide, has a radius of curvaturewhich is great, Compared to the wavelength k in the guide of the wave propagatng in the gude and is formed with a series of I a t wide teeth separated by 4 i wide ittervals` The above described device is designed either to be series inserted into a guide or connected thereto as a stub.

In such cases where the device according to the inventon s series inserted in the guide in which a millmeter wave is to be chopped at a given frequency, the device concerned is made up of a toothed sector, and the frequency, at which the wave is chopped, is equal to the frequency of rotation of the sector.

Where the device according to the invention is mounted as a stub, it consists of a toothed disc, and the frequency, at which the 'propagated millimeter wave is chopped, is equal to the frequency of rotation multiplied by' the number of teeth of the disk. v

The invention will be best understood from the following description and the attached drawings, wheren,

Figure 1 is an explanatory diagram of the Operating principle of the device according to the invention;

Figure 2 shows an elevation sectional view of a specific embodiment of the device according to the invention;

Figure 3 shows diagrammatically a circuit incorporating a device according to the invention;

Figure 4 shows diagrammatically a modification of th invention.

Referrng now to Figure 1, there may be seen in the latter a cross section of, for example a rectangular waveguide 1 for mllimeter waves, along a symmetry plane perpendicular to the larger walls of the guide. A slot 2 is provided, in the upper wall of the guide. An attenuator 3, consisting of a plane metal foil, of about millimeter thickness, penetrates into the guide 1 through the slot 2. Means are provided for keeping the attenuator 3 in the symmetry plane of the guide 1. That portion of the attenuator, which penetrates into the guide, displays along its edge a series of wide teeth, A being the wavelength in the guide of a millimeter wave which is assumed to propagate therein. The distance between the teeth is also If the number of teeth is great enough and if the teeth project deeply enough into the guide, the attenuator, as

seen from the source, constitutes a purely reactive impedance.

Assuming that Z is the characteristic impedance of that portion of the waveguide which does not include the slot, when a tooth 4 penetrates into the guide, it

forms an obstacle therein, with the result that, in that front face of the first tooth, provides the impedance Since it has been assumed that Z Z Z may be considered substantially equal to zero if n is great enough.

Thus the obstacle 3, as yiewedrom the first tooth, constitutes an impedance of zero value, hence a reecting obstacle, if n is great enough.

Experience shows that this is true when n is greater than 5.

It will be interesting to see what happens if a Shutter, or attenuator, S is 'rotated -about an *aiis 6, as shown in Fig. -2.

As long as the teeth 4 of the shutter 5 do not protect into the guide, the impedance of the latter is equal to the charcteristic impedance Z This impedance tends to approach zero as soon as a tooth pnetrates into the slot 2 and'is-restored to a value equal to the characteristic impedance Z when the teeth are driven out of the slot. Such a device will thus chop, or -amplitude-modulate to 100%, the energy propagati'n'g along the guide 1, at the frequency of the rotation 'ot the shutter 5.

Now, 'it is of interest to modulate millineter waves, at frequencies of the order of periods per second, in order to detet the existence of 'such waves. Gas cells are used for this purpose, having as their main constituent a vibrating membrane whose natural frequency ranges about 50 periodsper second. T herefore, the use of these cells involves modulation of the wave at a lower frequency.

According to Fig. 2,'the wave guide portion 1 has been provided with two fianges 7 'and 8, which enable that portion of the guide 1 to be inserted in a transmission line. i

Figure 3 shows diagrammatically a millimeter wave detecting circuitusing the device of Figure 2.

A millmeter wave generator 10 'feeds a guide 11, in which is inserted a wave guide portion 12, as shown in Fig. 2. An impedance matching system 13 is used to match the guide to a detecting-gas cell 14.

Figure 4 shows a diagram of another device for detecting the presence of `millimeter waves.

In this figure, the device according to the inventon is mounted as a stub 17 -which is coupled in series with a guide 19, wheren is propagated a millimeter wave, generated by a source 15 which feeds a load 20. In this case, the attenuator of the device 18 is no longer a sector, but actually a toothed disc whose teeth are shaped and spaced as explained above.

An attenuator 16 is inserted in the guide 19, between the stub *18 and the generator 15, thus preventing the energy, liable to be refle'cted 'toward the generator 15, from reaching the latter.

The operation of such a 'device is readily understandable with reference to Figur l.

It is clear that the number of teeth of the wheel 18 engaged= in the -slot 2 variesby nomore than one tooth viewed from the front of the first tooth,` the impedance of the wheel 18 is zero, this front moving along a distance equal to I t is as though a short-circuiting piston were moving nsde the guide, alonga travel equal to h di ta ce betwe ui e 39 and h lo in h guide 17 is equal to a whole number of slot. It may he assumed, and thisis confimed experimentally, that when a tooth is half engaged into the slot, the effect it exerts on the behavior of the whole assembly is negligible.

By means of the above system, the wave propagating from the source 15 towards the load 20 is or nearly 100%, modulated at a frequency F=n N, n being the number of teeth and N, the rotational speed of the disc 18 in revolutions per second. If the number of teeth is great enough, it will thus be possible to modulate the energy at a much higher frequency than with the previously described modification.

The invention may be used in many fields and is of course not limited to the described examples.

What I claim is:

1. A system for 100% modulating, or chopping, millimeter waves 'comprising: a wave guide portion for said waves; a movable metallic plane attenuator positioned in a plane of symmetry of said guide portion and having an edge in the shape of teeth separated by intervals and adapted for projecting at least five teeth, at least ,in certain positions thereof, into said guide; the teeth and the intervals having respective lengths equal to and the traverse dimension of said attentuator being of the order of compared to x, k being the wavelength in the guide of said Wave.

2. A system for 100% modulating, or chopping, millimeter waves comprising: a wave guide portion for said waves; a rotatable metallic plane attenuator positioned in a *plane of symmetry of said guide portion and with an edge having teeth separated byintervals and adapted for projecting at least five of said-teeth,` at least in 'certain positions of said edge, into said guide; said teeth and said intervals having respective lengths equal to and the transverse dimension of said attenuator being of the order of %o Compared to k being the wavelength in the guide of said wave.

and the transverse dimension of said attenuator being of the order of compared to k k being the wavelength in the guide of said wave.

4. A system for 100% modulating, or tchoppng, millimeter waves, propagating `in a rectangular wave guide having small and large walls, comprising: a wave guide portion for said wave, having a slot in the plane of symmetry of said large walls; means for connecting said portion as a stub to said guide, the distance between said slot and said guide being equal to n n being an integer; a rotatable metallie plane attenuator in the shape of a toothed disc adapted for projecting at least five teeth during its rotation into said slot; said teeth and the intervals therebetween having respective lengths equal to and the transverse dimension of said attenuator being or the order of compared to A., being the wave length in the guide of said wave.

References Cited in the file of this patent UNITED STATES PATENTS Nyswander Mar. 29, 1945 

